Use FilterComboBox instead of regular QComboBox

This required the following adjustments:

Instead of having a dedicated entry indicating the empty selection the
setPlaceholderText() mechanism is used.
Thus the locations where the 1st entry was assumed to be the empty
selection would have to be adjusted.

Additional checks for the empty string / index -1 have been added.

FindIdxInRagne() was adjusted to return -1 instead of 0 in case the
given string was not found.

Switched to index based singal instead of text based signal to be
notified about selection changes.

src/utils/transition-selection.hpp

@@ -1,8 +1,7 @@
 #pragma once
+#include "filter-combo-box.hpp"
 #include "utility.hpp"
 
-#include <QComboBox>
-
 namespace advss {
 
 class TransitionSelection {
@@ -28,7 +27,7 @@ private:
 	friend class TransitionSelectionWidget;
 };
 
-class TransitionSelectionWidget : public QComboBox {
+class TransitionSelectionWidget : public FilterComboBox {
 	Q_OBJECT
 
 public: